1. Field of the Invention
The present invention relates to a heat sink for electronic instruments, and more particularly to a high efficiency isothermal heat sink.
2. Description of Related Art
Integrated circuits (IC) are extensively used for various kinds of electronic instruments. Although power to drive the ICs comprising chips is very small, heat is generated when the integrated circuits operate. In high performance integrated circuits, such as central processing units (CPU) or video integrated circuits for computers, heat rapidly increases when the ICs operate. The increasing heat must be removed quickly from the ICs to keep the ICs from being damaged.
Heat sinks are used extensively to dissipate unwanted heat from operating ICs. Many techniques exist to make heat sinks quickly transfer heat from operating integrated circuits. Heat pipes have been used to cool integrated circuits for electronic instruments for decades.
With reference to FIG. 4, a conventional heat sink that uses the principles of the heat pipe in accordance with the prior air comprises a hollow plate-like body (60), a wick (61) and external fins (70). The body (60) has a top (not numbered), a bottom (not numbered) and an inner chamber (601). The inner chamber (601) is under a vacuum and has an interior surface (not numbered). The wick (61) is a capillary structure and is mounted on the interior surface of the body (60) to create a capillary action. The wick (61) may be a porous  structure made of materials like steel, aluminum, nickel or copper in various ranges of pore sizes or a mesh structure. The wick (61) is fabricated using metal foams or powder metallurgy. Fibrous materials, like ceramics have also been used widely, today.
The wick (61) has a condensing section (not numbered) and an evaporating section (not numbered) and contains a working fluid (not shown), generally pure water. When the heat sink is attached to a heat source (80) to dissipate heat from the heat source (80), the evaporating section is positioned adjacent to the heat source (80), and the condensing section is opposite to the evaporating section. The wick (61) transports the working fluid from the condensing section to the evaporating section by capillary action. The wick (61) must also be able to distribute the working liquid around the evaporating section to any area where heat is likely to be received by the heat sink.
As the heat source (80) heats the evaporating section of the wick (61), the working fluid in the evaporating section of the wick (61) evaporates and forms a hot vapor. Heat being absorbed through the bottom of the body (60) and dissipated through the top of the body causes the hot vapor (straight arrows) to flow toward the top of the body (60). Heat dissipated from the vapor is conducted to the fins (70) through the top of the body (60) and dissipated to the environment. The vapor cools and condenses to liquid that is absorbed by the wick (61) and is returned to the evaporating section by the capillary action of the wick (61) so that the working fluid is recycled. The working fluid in the evaporating section of the wick (61) is evaporated again, and the whole cycle is repeated to transfer heat from the heat source (80) for cooling. 
The fins (70) are mounted on the top of the body (60) adjacent to the condensing section of the wick (61) to dissipate heat from the condensing section of the wick (61).
However, the vapor and the returning working fluid in the wick (61) are in the same inner chamber (601), which causes a low efficiency of heat exchange in the inner chamber (601) for the conventional heat sink. Additionally, because the inner chamber (601) is kept at a vacuum, and the heat sink does not have any support between the top and the bottom of the body (60), the top and the bottom are easily drawn inward by the vacuum in the inner chamber (601). The deformed top and bottom of the body (60) cannot efficiently contact with the heat source (80) and the fins (70), which further lowers the efficiency of the heat sink.
To overcome the shortcomings, the present invention provides a high efficiency heat sink to mitigate or obviate the aforementioned problems.